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As police continue to search for whoever may have killed as many as 10 people, four of them prostitutes, on New York's Long Island, details are slowly beginning to emerge about where the bodies were found and how the victims were slain. Louis B. Schlesinger, a professor of forensic psychology at John Jay College of Criminal Justice at the City University of New York, has studied the minds of killers for decades. He recently completed a major research study with the FBI that examined the crime-scene behaviors of some 37 serial sexual murderers in relation to 162 victims. Schlesinger talked to TIME about the gritty details of the Long Island killings, how the public perception of serial killers often differs from reality and why the killer may have made a taunting phone call to the sister of one of his victims.

Who are police likely dealing with here?
This is a serial sexual murderer. I think it is one. Now, there is a report that they may be operating with someone else. That's possible, but it's very, very rare. It's so rare that you can count these guys on your right hand. And usually when there is more than one person, it's a dominant person who brings an assistant along. These are solo events. These individuals that are doing this do this very privately and very much by themselves. Because, don't forget, this is an expression of their sexual arousal pattern. These are sexual murders  the killing, the control and the domination are eroticized. With these guys there's a fusion of sex and aggression, so that the aggressive act itself is sexually stimulating. And that's why it's repeated.
(Watch TIME's video on truck-stop killers preying on prostitutes.)

Which is what makes these killings not only very scary but also very intriguing to the general public, no?
I think everybody can understand murder  it's not hard to understand. The typical motive for murder is anger. The prototype of murder is the Cain and Abel murder case from the Bible. Two people that know each other get into an argument, usually over jealousy, and someone is killed by a direct, violent assault. That's what most murders are. But sexual murder is very difficult to understand. The motivation is very different. The motivation is sexual and he's killing people that he doesn't know, that he has no beef with, and so this is something average people can't understand. You don't even know this person and you're killing them. And so it's a very different motivational pattern.

Why prostitutes?
Prostitutes are very typical targets of serial sexual murderers. The main problem that serial sexual murderers have is the act of abduction, getting the woman to go with them. That problem is eliminated with a prostitute because they'll go with anybody  it's part of their job description. And so that's why they're frequent targets.
(See the top 10 manhunts.)

Why has the location of these murders attracted so much popular attention? Is there a difference between urban and suburban or rural killers?
There's no difference. That is simply where the person lives or where he or she is comfortable. People kill where they have familiarity and comfort. So if somebody is killing in the Long Island area, they have comfort in that area. They know the area. They don't live in Pennsylvania and then commute to Long Island to kill somebody. That almost never happens. And even in serial-killer cases with very high numbers of victims, say, 30 victims, they all live or work in the same area. So if you're killing in an urban area, it's because you live there and you're familiar with it. If you kill in a rural area, it's because you're familiar and comfortable in that area as well.

There have been reports that a man  perhaps the killer  called the sister of one of the victims. If true, what does that tell us about the perpetrator?
These guys are extremely sadistic. Inflicting pain on others is sexually arousing to them. That's what sadism is, it's total domination and control over others. What these guys will do is they'll attach a ligature to somebody, strangle her until she's just about ready to die, loosen it so that she lives and then do it again to prolong sexual pleasure. Calling a relative of someone you just killed is also highly sadistic. There was an interesting New York case back in the 1920s; it was a notorious case of a guy named Albert Fish who befriended a family and was able to abduct their 12-year-old daughter named Grace Budd. He told the family that he was taking the daughter to a birthday party for his niece. And he took her to a house in Westchester County, where he not only killed her, but chopped the body up and ate her, with potatoes and vegetables in a stew, over a 10-day period. They never made an arrest, at that point. I think it went on for close to 10 years, but what happened was Fish sent a sadistic letter to the mother of the child, and that was how he got apprehended.